1. Field of the Invention
The present invention relates to a method and apparatus for particle separation.
2. Present State of the Art
The invention particularly relates to the separation applied in the mining, extracting field and the like, wherein the particles to be separated, obtained from minerals reduced to a predetermined size, are dispersed into a fluid fed to dynamic separators, both conical ones generally called as cyclones, and cylindrical ones or combinations of both shapes.
The latter are machines exploiting centrifugal forces that are generated by providing a rotational motion to a fluid containing the suspended particles, inside a chamber preferably having cylindrical or frustoconical geometry.
Thus the field of centrifugal forces generated into the (gaseous or liquid) fluid causes the particles to be arranged in layers with the same terminal velocity, which depends on the density, size, shape, etc., concentric to each other, thereby causing the particles with lower terminal velocity to float along the axis of the separator, and the particles with higher terminal velocity to sink by entraining them against the walls of the cylindrical or frustoconical shaped chamber.
In the mining industry, the separation cyclones are apparatus typically used for separating the extraction material, after crushing and grounding the minerals into granules of predetermined size.
Commonly called cyclone separators have mainly a frustoconical shape and are equipped with a tangentially arranged inlet duct and with two outlets along the axis of the separator. In particular a first outlet is arranged near the vertex of the cone and it is intended to discharge the fraction of heavier particles or more in general those having higher terminal velocity, while the second one is arranged at the base of the cone and serves for discharging the fraction of particles having lower terminal velocity.
Dynamic separators with a mainly cylindrical shape are usually used for a more accurate separation than cyclone ones. If the desired separation is mainly due to the density of the particles, a dense medium with a predetermined density is used; such separators can have, depending on needs, a preferably cylindrical or frustoconical shape. These apparatus are part of the prior art that has been established for many years.
The cyclone separators and the dynamic cylindrical ones mentioned above have been well-known since a long time and in order to improve their performances, the fact of arranging them in series one after the other (both conical and cylindrical separators) is also known to obtain multi-stage separation apparatuses.
Examples of such apparatuses are described in Italian and English patent publications IT 1152915 and GB 2 164 589, both to Prominco S.r.l.
The operating principle of multi-stage apparatuses is that by dividing the separation into several successive stages, it is possible to have for each one of them better conditions to remove the light fraction from the heavy one, since the probability increases that a particle be discharged into the e stream of which it is part.
Indeed as shown in FIG. 1 annexed hereto and taken from the already mentioned document GB 2 164 589, a single separation stage (curve A) is able to remove a smaller percentage (under equal conditions) of a material having a predetermined density, than that obtainable by increasing the number of separation stages (curves B,C,D): thus it is possible to increase the accuracy of the separation therefore enhancing the performance of the whole process.
A particularly interesting case is when a high quality coal is desired (density of 1.1-1.3 kg/dm3).
There are cases when three or more by-products have to be divided (for example metallurgical coal, thermal coal and tailings) from a feeding material; to this end, it is also possible to advantageously use dense media with different densities in the several stages instead of using completely independent separation stages which are much expensive to be installed and operated, however making the connection with each other not favorable if the difference between the two media is very high (e.g. d1=2.00 kg/dm3, d2=1.45 kg/dm3). As a matter of fact, in this situation a difference in the density between the fluid entering the following downstream separator and its dense medium (e.g. i1=1.8 kg/dm3, d2=1.45 kg/dm3), would be generated, such that the separator would operate under not optimal working conditions at least in a part of the next stage of the separator, thus accomplishing a low quality separation or requiring the stage to be oversized.
As it can be noted, on one hand the use of a greater number of separation stages theoretically allows the performance of the separation process to be optimized (as regards the separation accuracy if only two products are separated or as regards the cheapness if three or more products are separated) on the other hand apparatuses are made more complicated which in practice means that they are functionally little flexible, since in order to keep in the several stages the design conditions for the separation it is not possible to make changes in one stage without affecting the following ones.
Consequently, if for any reasons should the conditions of the fluid (e.g. flow rate, density, etc.) with suspended particles change into a stage of the separator, even the other ones would be involved worsening the performances of the whole apparatus.